1. Field of the Invention
The present invention is related to a filter layer for displays, a method of preparing the same and displays including the same and, more particularly, to a light absorbing filter layer for improving contrast and color coordinate ranges of displays, a method of preparing the same and displays including the same.
2. Description of the Related Art
A cathode ray tube (further referred to as CRT) is one of the present major image displays. As large display and high-resolution televisions are in demand, a light and thin flat panel display (FPD) with improved brightness has been developed actively. Examples of the FPD, are a liquid crystal display (LCD), an electroluminescent display (ELD), a field emitter display (FED), a plasma display panel (PDP) and so on.
The CRT is a display for color images that emits stripe-type or dot-type red (R), green (G) and blue (B) phosphors of a phosphor screen on which the electron beams radiated from an electron gun collide. The phosphor screen is prepared by forming phosphor layers between light-absorbing black matrix layers on a face panel.
FIG. 1 illustrates a partial cross-sectional view of the face panel with a coated phosphor layer of a conventional CRT. A conventional CRT, as illustrated in FIG. 1, for example, includes two sources of visible light coming out of the face panel. One is a light L1 emitted from phosphors (R,G,B) when electron beams impinge on them. The other is external ambient light reflected from the face panel 10. The reflected light has in turn two components depending on where the incident external light is reflected. A first component L2 is reflected light on the surface of the face panel 10. A second component L3 is the light that passes the face panel 10 and then is reflected off at the interface of the phosphor screen 2 and the inner surface of the face panel 10.
As the CRT is designed to emit light at only predetermined wavelengths and to display a color image by a selective combination of these predetermined wavelengths, the ambient light reflected from the face panel has a uniform continuous spectrum and has different wavelengths from the predetermined wavelengths, thus degrading the contrast of a CRT.
FIG. 2 illustrates spectral luminescence curves of P22 phosphor materials commonly used in the art. Blue phosphor ZnS:Ag, green phosphor ZnS:Au,Cu,Al and red phosphor Y2O2S:Eu have their peak wavelengths curves 21 to 23 of FIG. 2 at 450 nm, 540 nm and 630 nm, respectively.
The light components L2 and L3, reflected from external ambient light have relatively higher illumination between these peaks 21 to 23 of FIG. 2, since their spectral distribution is continuous across all the visible wavelengths. The spectrum of light emitted from the blue and green phosphor has relatively broad bandwidths and thus some of wavelengths, from 450 nm to 550 nm, overlap with each other. The spectrum of red phosphor has undesirable side bands around 580 nm, at which wavelength the luminous efficiency is high. Therefore, selective absorption of light in the overlapping wavelengths between blue and green phosphor at and around 450 nm to 550 nm would greatly improve color purity of a CRT without sacrificing the luminescence efficiency of phosphors.
Also, because absorption of light around 580 nm makes the body color of a CRT appear bluish, external ambient light around 410 nm is preferably made to be absorbed in order to At compensate for the bluish appearance.
Efforts have been made to find a way to selectively absorb light around 580 nm, 500 nm and 410 nm in order to provide a CRT with improved brightness. For example, U.S. Pat. Nos. 5,200,667 to Iwasaki et al., 5,315,209 to Iwasaki and 5,218,268 to Matsuda et al. disclose forming a film including dyes or pigments that selectively absorb light on a surface of the outer surface of the phosphor screen. Alternatively, a plurality of transparent oxide layers having different refractive index and thickness have been coated on the outer surface of a face panel to take advantage of their light interference for the purpose of reducing ambient light reflection. However, there is also a need to reduce light reflected at the phosphor layer and at the inner surface of face panel.
In relation to the problem as described above, U.S. Pat. Nos. 4,019,905 to Tomita et al., 4,132,919 to Maple and 5,627,429 to Iwasaki relate to an intermediate layer including organic or inorganic pigments or dyes with absorbability of light at predetermined wavelengths that is coated between the inner surface of the face panel and the phosphor layer. While such a technique can be advantageous with respect to the application of a manufacturing process of a CRT, the dyes and pigments used in the intermediate layer typically have a broad absorption wavelength and, thus, the contrast of the CRT generally does not improve significantly.
Also, U.S. Pat. Nos. 5,068,568 to de Vrieze et al. and 5,179,318 to Maeda et al. disclose an intermediate layer including layers of a high refractive index and a low refractive index alternately between the inner surface of the face panel and the phosphor layer. Further, a method of forming a corresponding filter layer on an RGB phosphor layer is described in SOCIETY OF INFORMATION AND DISPLAY DIGEST, “5.1 Invited Paper: “Microfilter”™ Color CRT”, Itou et al., 1995 pages 25-27. However, this method typically needs additional equipment and a modification of the manufacturing process, since coating, light exposing and developing processes for the corresponding filter layer are typically further conducted compared to a conventional technique.
Additionally, U.S. Pat. No. 6,090,473 to Yoshikawa et al. discloses a plasma display panel including a face panel to which a glass plate or film is adhered so as to improve contrast and shield an electron wave.